Re: getrandom: Fail with ENOSYS when the system has no randomness source

[Paul Eggert]

On 5/9/21 7:51 AM, Bruno Haible wrote:
>         if (fd < 0)
> -        return fd;
> +        {
> +          errno = ENOSYS;
> +          return -1;
> +        }

If 'open' fails with errno equal to (say) EINTR or EAGAIN or EMFILE, 
this doesn't mean the operating system lacks a randomness source; it 
merely means the 'open' failed. And the libc manual allows getrandom to 
fail with EMFILE or with any other valid error number. (FWIW, the Hurd 
implementation of getrandom simply passes the errno of 'open' through.)

How about if we instead change that code to something like this:

 if (fd < 0)
   {
      if (errno == ENOENT || errno == ENOTDIR)
        errno = ENOSYS;
      return fd;
   }

That is, if /dev/random (or whatever) doesn't exist, we assume the OS is 
like IRIX and lacks randomness support, so we fail with ENOSYS; 
otherwise we pass errno through as that's more useful to the caller.